Sub-bituminous coals such as that of Wyoming's Powder River Basin as well as coals of lower rank such as brown coals and lignites are characterized by substantial water contents which can fall in the range of 25% to 40%, by weight. Such a high water content inhibits the use of these coals as fuel by reducing the heating value and increasing transportation cost. It has accordingly been considered desirable to dry such coals prior to shipment.
The problem has been discussed in U.S. Pat. No. 4,401,436 as follows:
The drying required with such low rank coals is a deep drying process for the removal of surface water plus the large quantities of interstitial water present in such low rank coals. By contrast, when higher grade coals are dried, the drying is commonly for the purpose of drying the surface water from the coal particle surface but not interstitial water, since interstitial water content of the higher rank coals is relatively low. As a result, short residence times in the drying zone are normally used, and the interior portions of the coal particles are not heated, since such is not necessary for surface drying. Typically, the coal leaving the dryer in such surface water drying processes is a temperature below about 110.degree. F. (45.degree. C.). By contrast, processes for the removal of interstitial water require longer residence times and result in heating the interior portions of the coal particles. The coal leaving a drying process for the removal of interstitial water will typically be at a temperature from about 130.degree. to about 250.degree. F. (54.degree. to 121.degree. C.). When such processes for the removal of interstitial water are applied to low rank coals, the resulting dried coal has a strong tendency to spontaneously ignite, especially at the high discharge temperature, upon storage, during transportation and the like. As a result, a continuing effort has been directed to the development of improving methods whereby such lower grade coals can be dried and thereafter safely transported, stored and used as fuels.
It has been found, however, that the process of heating the coal to remove water and then cooling the heated coal produces a substantial quantity of fines, i.e., particles having a mean average particle size less than 8 mesh.
Sub-bituminous coal is usually crushed to a top size 3/4" to 1-1/8" before drying. Some production of fines occurs on crushing. During the drying and cooling operations, attrition of the coal occurs in the fluid bed as particles are in contact in a moving gas stream and decrepitation of coal particles occurs during contact with the hot gas stream, which stream may be heated to a temperature on the order of 900.degree. F. Exposure to the hot gas causes vaporization and expansion of gases within the coal particles, an effect which also results in particle breakdown. The accumulation of physical, thermal, and possibly, chemical effects result in undesirable particle breakdown and production of excessive amounts of fine material.